1. Field of the Invention
The present invention pertains to a device for attaching tension test accessories to test machine frames in materials testing. A spherical seat inside of the device allows the test accessories to self-align as the test commences in order to assure that only tension force is applied during the test.
2. Description of the Prior Art
The prior art has included tension rods for material testing machines which use a long section of threaded rod inserted through the grip pocket of a test machine crosshead. On the side of the crosshead facing away from the tension test space, a plate is attached using fasteners. This plate is machined to accept the lower half of a spherical washer assembly. The upper half of the spherical washer sits against the lower half and the whole assembly is held together by a nut on either side of the test machine crosshead. However, tension rods that mount inside of the grip pocket require that the machine have grips enclosed inside of the crosshead. Such machines are typically dual test space machines, which offer a separate test space for tension and for compression and tend to be large and expensive. Many single test space machines are unable to use in-head tension rods. Additionally, installing the tension rods requires that the grip jaws and related parts are removed, which is a sensitive process requiring a specific method of lubrication during reinstallation. The multiple components make the device complicated, expensive and labor-intensive to install and adjust. Additionally, the multiple parts of the spherical washer and tension rod require many high precision concentric surfaces to be machined, adding uncertainty to the concentricity of the assembly.
The prior art has likewise included threaded spherical tension couplings having a protruding male threaded section intended for the test machine and a female threaded section intended to duplicate the threaded interface of the test machine. The coupling has a cylindrical section between each end inside of which the spherical seat is located, placing it well within the test space. Such an externally seated, male-female spherical coupling has the primary disadvantage of placing the spherical seat inside of the test space, which is undesirable for best tension test results and in that it consumes test space. Additionally, such couplings have typically had a large spherical radius (or radius of curvature) which resulted in significant resistance to self-alignment.
The prior art, in order to provide tension rods for a test machine already fitted with wedge grips, has further included a spherically seated tension rod device that is held in the grip jaws, similarly to the way in which the grip jaws would grip a specimen. The section held within the grip jaws is solid, and the spherical seat is contained within a cylindrical section located within the test space. From this point, the tension rod protrudes further into the test space. However, this configuration is disadvantageous in that it requires a constant grip force to stay in place. During machine testing, this requires machine power and adds an element of uncertainty to the test results. When a test machine is powered down, the grips may loosen and allow the tension rod assembly to fall out, creating a safety hazard that must be prevented by adding a retainer plate underneath. This plate must then be removed along with the tension rods. Moreover, these tension rods are used on test machines that already have a limited test space due to the grips. A spherical seat extending even further into the test space has made these unfeasible for many applications.